@Stuartw Regarding the C - Am - Em - G progression and knowing which key it’s in, you need to know (or at least be aware of) several things. The first is the major scale formula with which you can find out the notes of a given major scale. Then you need to know the nature of diatonic chords within a certain key, i.e. the (3-note) chords that can be created from the notes of the given scale (and only those notes).
Below, there’s a small diagram I’ve just made in Excel showing the keys that this C - Am - Em - G progression can be in, with the chords highlighted in orange. The lone non-diatonic progression is the only one I could think of off the top of my head as non-diatonic progressions are not in the focus now. Relative minor keys are not discussed, either.
Your first guideposts should be the major chords, C and G. In the diagram I highlighted the scale degrees where major chords can fall in green; these are degrees I, IV and V. They are major chords because they contain the major third in relation to the major scale of their root note.
You can see that two of these major degrees, IV and V, are adjacent, i.e. the root notes of these major chords also have to be adjacent. But C and G are not adjacent, so they cannot be the IV and V chords. That leaves us with the option of one of them being I and the other being IV or V.
Now, you can either write out the two keys as I did, or you can examine the interval relationship of C and G (the notes). And this is where another key skill, knowing the notes and intervals on the fretboard, comes into play. What you can see in the diagram written out in letters and numbers can also be seen on the fretboard in shapes. Which notes do you have at fret 3 on strings 6 and 5? That’s right, G and C. C is a perfect fourth above that G, and G (or its octave, to take the pitch difference into consideration) is a perfect fifth above C.
Now on to the two minor chords, Am and Em. In every major key, the ii, iii and vi chords are minor because they contain the minor third in relation to the major scale of their root note.
Again, you have two adjacent degrees, ii and iii, but A and E are not adjacent, so they cannot be the ii and iii chords. One of them must be vi and the other has to be the ii or iii. Writing out the scales is still an option, but checking the interval between A and E is not difficult either. Since there are two open strings of those notes, you don’t need to worry about the fret position. The interval distance between A and E is the same as the interval distance between C and G: it’s a perfect fourth/fifth relationship. This is what I tried to highlight with the arrows in the diagram below. (Imagine arrows from Am to Em and C to G in the same lines, too.)
What remains is to fit these chords into the diatonic chord formula and check their degrees. You can see this under “Chord progression”.
So, you need 3 things:
- Know the notes and intervals on the fretboard
- Know the major scale formula
- Know the diatonic chords of the major keys
Without these, it’s like trying to run a marathon without having learned to walk first. If you have subscribed to the practical music theory course, make sure to spend time on the first 3 grades until you get comfortable with these topics. There’s no need to rush things.
